The long-term goals are to assess the biological consequences of modified DNA bases. The research represents a broad based program employing enzymatic, chemical, genetic, and molecular biological approaches to examine in detail the production and repair/processing off ionizing radiation damage. Substrate specificity studies as well as cloning and wequencing of Escherichia coli repair enzymes are planned. These enzymes will also be used as probes for X-ray-induced DNA damage. Further, unique modified bases are being examined for their ability to serve as replicative breaks in vitro and lethal and/or for mutagenic lesions in vivo. In addition, the effect of various E. coli repair proteins on the processing of unique lesions will be measured by transfecting phage DNA containing these lesions into E. coli mutants defective in particular DNA repair-associated gene products. Lastly, the function of the bacteriophage T4 uvsY and uvsW genes, that are involved in genetic recombination and error prone mutagenesis, are being studied. Ionizing radiation produces a spectrum of DNA damages and the present studies are designed to examine the potential of unique lesions to produce lethal, mutagenic and therefore presumably carcinogenic endpoints.